Femtocells are small cellular base stations that are, for example, being offered by network providers and others. A femtocell is intended to constitute a part of a cellular network. Femtocells are usually implemented in order to provide enhanced communications coverage in a relatively small coverage area such as an office building, a home, a subway terminal, or the like. Unlike traditional cellular base stations, which are deployed and configured statically by the network operator, femtocells are generally smaller capacity cellular base stations that are intended to be self-configuring. In principal a user should be able to acquire one, place it within a cellular communications network, turn it on, and have it start working. To accomplish this the femtocell needs to be able to sense its environment with a radio receiver capable of receiving cellular communications network communications and dynamically configure itself in order to minimize interference (and maximize cooperation) with neighboring cells.
The way this is done is to have the femtocell use its radio receiver to periodically survey its environment in order to determine the characteristics of cellular communications networks to which it might attach. An initial survey occurs when the femtocell is first powered on, and then periodic re-surveys are carried out after that.
A first level of the survey is conducted in order to detect nearby base stations from which the Femtocell may obtain information about the cellular communications network. The goal of such detection is to determine the base stations that are present, and identify a few first-order configuration parameters for the femtocell. This step does not involve “drilling down” into signal internals (complete demodulation and decoding). It simply involves (in the example case of a UMTS/WCDMA network) identifying the scrambling code used by each base station heard, and the corresponding received power. This information can then be used to set first-order configuration parameters of the femtocell, such as scrambling code and transmit power.
A second level of the survey is subsequently carried out in order to attempt to recover System Information (“SysInfo”) broadcast by each of the base stations detected in the previous step. The SysInfo contains more detailed information useful to the configuring of a cellular base station operating in a cellular communications network. The recovery of SysInfo is done by demodulating each base station signal (converting it from an analog radio signal to a data stream, e.g., of 1's and 0's) and decoding the SysInfo into intelligible information. The femtocell can then use the SysInfo received from other base stations to refine its configuration in order to operate more seamlessly with surrounding base stations.
Demodulation and decoding of SysInfo is limited by signal to interference plus noise ratio (“SINR”) (i.e., thermal noise as well as interference from other base stations), so full SysInfo recovery is typically not feasible for many of the cellular base stations that may be detected. This is particularly true for femtocells as they are designed for home/building use, and the antenna(s) used for reception may not be situated optimally for the reception of signals from local base stations. This limitation unfortunately restricts the ability of a femtocell to optimally configure itself. The result is more interference among base stations, which ultimately translates to decreased network capacity and more dropped connections resulting in femtocells operating less optimally than possible.
Once the survey is complete, the femtocell configures itself as best it can with the information it has received, and then switches into operational mode. The survey is periodically repeated to insure that the femtocell is provided with current information about its network environment.